1. Field of the Invention
The present invention relates to an optical film, an anti-reflection film, a polarizing plate comprising the optical film and an image display device comprising the optical film.
2. Description of the Invention
An optical functional film is normally applied to image display devices such as cathode ray tube display device (CRT), plasma display (PDP), electroluminescence display (ELD) and liquid crystal display device (LCD). Examples of such an optical film include anti-reflection films, anti-glare films, light-scattering films, optical compensation films, and polarizing plates.
These optical films are normally prepared by a thin layer forming method involving vacuum evaporation such as sputtering or physical adsorption such as atmospheric plasma evaporation. However, a thin layer forming method involving coating can more easily meet the requirements for larger area and provide a higher productivity than the aforementioned thin forming methods. The demand for this thin layer forming method has been recently growing.
In accordance with the thin layer forming method involving coating, it is often practiced to provide a functional layer by spreading a solution, particularly a solution comprising an organic solvent, over a transparent support. However, since the aforementioned optical films are used in an image display device adapted for viewing, an extremely severe quality against coating defects such as coating streak and drying unevenness is needed. In particular, with the trend for larger image display device screen, even weak coating defects can be easily recognized. It has thus been required that there occur little or no coating defects.
As the transparent support for the aforementioned optical films there is normally used a cellulose acylate film, particularly cellulose triacetate film. The cellulose acylate film is normally produced by a solution film forming method or fusion film forming method. The solution film forming method can produce a film having a higher planarity than the fusion film forming method. For example, JP-B-5-17844 discloses a method of producing a cellulose acylate film. Practically speaking, the solution film forming method has been normally employed.
It is normally practiced to incorporate a plasticizer in such a cellulose acylate film for the purpose of controlling the flexibility and moisture permeability of the transparent support. JP-A-8-57879 discloses a method of controlling the surface plasticizer content.
Referring to the aforementioned cellulose acylate film, the cellulose acylate and the plasticizer can be dissolved in a solvent. Accordingly, in the case where an organic solvent is spread directly over the cellulose acylate film to provide a coat layer, the solubility of the transparent support in the coating solvent and the power of penetration of the coating solution into the transparent support are affected. JP-A-2002-169001 discloses a method of controlling the kind and ratio of solvents having different powers of dissolving the support to control the adhesion and optical properties in the case where an optical functional layer is spread over such a cellulose acylate film.
As mentioned above, in the case where a coating solution comprising an organic solvent is spread over a cellulose acylate film containing a plasticizer, the spreadability and dryability of the coating solution are affected by the affinity of the cellulose acylate or plasticizer for the organic solvent, the solubility of the cellulose acylate or plasticizer in the organic solvent, etc.
The coating solution normally contains components for forming an optical functional layer or physical functional layer. As a coating solvent, a solvent having a good power of dissolving the components and a good stability is needed. In the case where a coating solution comprising an organic solvent is spread, it is necessary from the standpoint of working atmosphere or residual solvents in the finished optical film that a solvent having little toxicity to human body and ecosystem and environmental safety problems.